1. Field of the Invention
The present invention relates generally to the field of optical waveguides, and more particularly to a high sensitivity filter and a method of filtering certain wavelengths of light utilizing a tunable optical waveguide.
2. Discussion of the Related Art
Detecting chemical substances in any environment is a difficult task. Spectroscopic techniques are by far the most prevalent and most utilized for investigating and detecting the presence of chemical substances. Though spectroscopic techniques are widely utilized, there are many associated problems and difficulties, especially when such techniques are needed and performed outside of a controlled laboratory environment. Known spectroscopic techniques are very sensitive to conditions of the measurement environment and are not particularly versatile. Therefore, practitioners bear the burden of enormous expense and logistical difficulties in order to utilize these known methods in field applications beyond the laboratory environment.
One example of a widely used method is known as correlation spectroscopy. In this method, the wavelength spectrum of a detection system or energy source is periodically modulated into and out of a spectrum match with characteristic features of the spectrum of a known target substance that is usually in a gaseous state. Such correlation spectroscopy systems are used to obtain critical information and data from a monitored environment. Specifically, correlation spectroscopy is used for monitoring the atmospheric contents at a given moment in time or in a specified space. To determine whether a particular chemical substance exists in an environment, a practitioner must have available a self-contained reference cell containing therein a quantity of the particular gaseous substance. If a practitioner is out in the field, and if the detection of several substances is desired, a reference cell must be available that can be evacuated and refilled separately with each different reference substance and a certain amount of each substance must be on hand. Alternatively, a number of reference cells each containing one of the particular substances must be available at the site. This method is extremely costly and inconvenient and requires careful handling to utilize such reference cells for field detection and experimentation applications.
Despite the cost and inconvenience, over the past 20 years such correlation spectroscopy systems have been widely used all over the world because of the reliable results that can be achieved. Particularly, long path reference cells have been transported along with very large gas-laser spectrographs in aircraft or by other modes of transportation in order to do testing in the field.
Looking to more recent technology, tunable semiconductor laser diodes can now be custom-engineered to elicit spectral ranges that match some of the light or energy absorbance characteristics of selected target chemical species. Tunable laser diodes are extremely expensive and are also difficult to transport, although the diodes do eliminate the need to transport and maintain the traditional reference cells and gaseous samples in order to conduct field evaluations. In addition, tunable laser diodes are typically only useful and accurate over a fairly narrow spectral range.
Other alternative types of tunable filters, for example, Fabry-Perot tunable filters, are known and sometimes utilized for chemical detection. However, no alternative type of tunable filter produces highly accurate spectral recording data. These alternative filters thus produce an unsuitable spectral match to a target compound absorbance range.
A typical correlation spectroscopy method compares the wavelength spectrum of light passing through a sample environment to be analyzed with the wavelength spectrum of a known target substance. The spectra can be compared optically in a number of different ways. One way is to split a light source into two separate paths with one path being passed through the sample substance and the other passed through the known reference or target substance. Once the light from the two paths is passed through the two substances, the light is re-combined. The spectral characteristics and intensity of the light in the two paths can be measured separately and compared to one another to determine the presence of and/or the concentration of the target substance within the sample. Alternatively, the light re-combined from the two paths can be measured and compared to the characteristics of the original light source prior to passing through the two substances and prior to splitting the light source.
To hone or enhance readings, the wavelength spectrum of light passing through the reference chamber or reference substance is periodically modulated or shifted during measurement. Modulation is done in order to minimize signal noise, to increase the sensitivity of the measurement, and produce a more precise reading. Modulation is often done for conventional correlation spectroscopy techniques by producing pressure changes, temperature changes or electrical field changes within the gas reference cell, or, for example, by changing a characteristic of a filter such as the spacing of a Fabry-Perot cavity. Utilizing these means to modulate the spectrum of light that passes through a reference cell or a currently known filter, however, cannot be done at sufficiently small or precise increments. Therefore, these techniques do not produce adequately sensitive readings. The sensitivity produced by these methods is not as high as is necessary to detect relatively low concentrations or small concentrations of a substance.
The present invention is for a method and an apparatus to overcome the above-described problems and other deficiencies associated with prior art chemical detection systems and techniques. The invention includes both a method and an apparatus that are much more convenient and less cumbersome to use for chemical substance detection both in a laboratory and in field applications. Additionally, the invention eliminates the necessity of transporting, refilling, storing and maintaining multiple reference gas cells. The invention also eliminates the necessity of transporting, refilling, storing and maintaining multiple gases and other chemical substances that are sometimes flammable, explosive or otherwise dangerous.
One object of the present invention is to eliminate problems created when a gas reference cell leaks, loses pressure, or changes concentration by undesired admission of air into the reference cell. A further object of the present invention is to eliminate the necessity of maintaining and transporting reference cells. These cells can often be fairly large where the particular reference substance has a wavelength range absorption with a long wavelength path. Another object of the present invention is to provide a method useful for correlation spectroscopy that is less costly to set up and perform, is less difficult and cumbersome to use, is easier to use in field applications, and produces more accurate results. An additional object of the present invention is to provide an apparatus that can be utilized for correlation spectroscopy that is less costly to produce and purchase, is less difficult to use, produces more accurate results, and is less costly and easier to transport and maintain in field applications.
A still further object of the present invention is to provide such a detection method and apparatus that can also be adapted and used for other purposes. One such purpose for which the present invention is particularly well suited is for calibrating a spectroscope in a highly accurate and simple manner. A further object of the present invention is to provide a method and apparatus useful in other fields such as, for example, differential absorption laser devices and distance array radar systems.
Devices and methods are disclosed herein to achieve these and other objects of the present invention. In one embodiment, the present invention provides a device that simulates the absorbance characteristics or wavelength spectrum of a desired substance upon passing light energy through the device. The device includes an optical waveguide having a core material, an input end, an output end, and a nominal core refractive index. The waveguide also has predetermined periodic variations in the refractive index formed in the core somewhere between the input and the output ends. The periodic variations change the nominal core refractive index of the waveguide producing regions of altered refractive indices that reflect a predetermined set of wavelengths. The periodic variations or altered refractive index regions are predetermined to produce characteristic wavelengths of a reference spectrum that match a target wavelength spectrum of a target or selected substance. The periodic variations or altered refractive index regions can be formed such that the characteristic wavelengths of the reference spectrum are produced either as filtered light passing to the output end or as light reflected by the periodic variations back to the input end. The periodic variations are intermittently modulated or shifted to fine-tune the reference spectrum.
In one embodiment, the device has a modulator coupled to the waveguide to intermittently modulate the regions of altered refractive index and hence the characteristic wavelengths of the reference spectrum of the waveguide.
In one embodiment, the device has a light energy source coupled to the input end of the waveguide. The light energy source can be any type of light source producing either a coherent or an incoherent beam of light.
In one embodiment, the device has an energy source coupled to the modulator to selectively apply energy to the modulator to intermittently alter a physical characteristic of the waveguide. Altering the physical characteristic in turn modulates the periodic variations of the waveguide in order to modulate the characteristic wavelengths of the reference spectrum.
In one embodiment, the waveguide is an optical fiber having a fiber core as the core material surrounded by a fiber cladding material. In another embodiment, the periodic variations are a Bragg grating formed in the core of the optical fiber having a plurality of spaced apart altered refractive index regions or individual gratings. In another embodiment, the modulator periodically alters the characteristic spacing and thus intermittently shifts the refractive index of the gratings to modulate the characteristic wavelength of the reference spectrum of the gratings.
In one embodiment, the modulator is a cylinder or ring of material having an alterable circumference or diameter. The waveguide is wrapped around the circumference of the cylinder. The size of the circumference is modulated to periodically stretch the waveguide at the periodic variations to modulate the characteristic wavelengths of the reference spectrum of the waveguide.
In one embodiment, the modulator includes a planar substrate having the waveguide attached or fixed to one surface. A physical characteristic of the one surface is intermittently altered which in turn modulates the periodic variations and thus the characteristic wavelengths of the reference spectrum of the waveguide. The modulator may vary within the scope of the invention and other modulator constructions are described herein.
In another embodiment of the invention, a method of producing a very accurate and predetermined reference wavelength spectrum is disclosed. The method includes providing an optical waveguide having a core material, an input end, an output end, and a nominal core refractive index. Predetermined periodic variations in the refractive index are added to the core material of the waveguide to produce regions of altered refractive index in the core material. The periodic variations produce a reference spectrum having characteristic wavelengths. A source of light energy having known spectral characteristics is coupled to the input end of the waveguide. The spectral characteristics are monitored either at the output end of light energy after passing through the waveguide or at the input end of light energy after being reflected by the periodic variations back to the input end. The periodic variations are modulated to intermittently modulate the characteristic wavelengths of the reference spectrum. This modulation enables signal processing techniques that reduce signal noise of the monitored light energy.
In one embodiment, the method of the invention is used for correlation spectroscopy in detecting the presence of a substance in a test or sample environment. The periodic variations in the refractive index of the tunable filter are tailored so that the characteristic wavelengths of the reference spectrum match the optical absorbance spectrum of a selected or target substance.
In one embodiment, an identical light energy source is passed through both the tunable filter waveguide and the sample environment. The waveband spectrum characteristics of the light energy exiting both the tunable filter waveguide and the sample environment are monitored and compared to determine the presence or absence of the selected substance. If the characteristic wavelengths of the reference spectrum and a portion of the target spectrum of the sample environment match, the substance is known to be present in the sample environment.
In one embodiment, a light energy source is passed first into the tunable filter waveguide designed to match the selected substance. Light energy reflected back by the periodic variations is then passed through the sample environment. The light energy emerging from the sample environment is then monitored to determine the presence or absence of the selected target substance. If the characteristic wavelengths of the reference spectrum are absorbed in the sample environment, the substance is present in the test or sample environment.
In one embodiment, the step of modulating is achieved by fixing the waveguide to a substrate and intermittently altering a physical characteristic of the substrate to selectively shift the periodic variations in the refractive index to intermittently alter the characteristic wavelengths of the reference spectrum of the waveguide. The step of modulating can be conducted in many different ways and yet fall within the scope of the invention.